Calculation of Cosmic-ray Ages in the Iron Meteorites ‘Carbo’ and ‘Treysa’

Abstract

IN a recent communication¹, Dr. R. R. Marshall estimates the cosmic-ray age of the iron meteorite ‘Carbo’ to be 0.69 × 10⁹ yr. His calculation was based upon the following experimental results of ours² and Barr's³. We showed that the relative isotopic abundances of potassium from a sample of the iron meteorite ‘Carbo’ were markedly different from those of terrestrial potassium. Considerable enrichment of the isotopes of masses 40 and 41 was found (³⁹K_m = 79.5 per cent, ⁴⁰K_m = 4.8 per cent, and ⁴¹K_m = 15.7 per cent as compared to 93.08, 0.012 and 6.91 per cent respectively for ordinary terrestrial potassium). Evidence was given that a considerable amount of this meteoritic potassium—nearly all the potassium-40 and more than half the potassium-41—is of cosmogenic origin, that is, it is a reaction product of the spallation of mainly iron nuclides, induced by high-energy particles from cosmic radiation. Barr³ measured the cross-sections for the production of fifty-nine radioactive isotopes formed by 5.7-BeV. proton bombardment of copper. From his summary of measured and interpolated independent cross-sections, Marshall calculated the total cross-sections for the production of the potassium isotopes to be σ₃₉ = 14.8 mb., σ₄₀ = 9.4 mb., and σ₄₁ = 14.7 mb.